In the previous section, we have seen how to select external capacitors of a crystal based on parasitic and load capacitance of crystal. The selection of these capacitors, determine the frequency of operation. In this section let us see what the individual parts of crystal section are going to contribute to oscillation.
- Inverter inside the processor/micro controller acts as Class AB amplifier
- Inverter provides 180 degrees phase-shift
- For oscillation to be stable, the phase-shift must be 360 degrees
- Additional 180 degrees phase-shift is provided by C1, C2
- As per oscillation theory, closed loop gain must be greater >= 1 for oscillation to be maintained
- Loop gain gets reduced by ESR, shunt capacitance and load capacitance
- Select a crystal with load capacitance <= 12pF
- More the load capacitance of the crystal, more the power dissipated
- If frequency of oscillation is higher than desired, increase load capacitance to reduce it to desired level
- If frequency of oscillation is lower than the desired, reduce the load capacitance value to get desired frequency
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